A New TB Vaccine
Scientists from Oxford University have formulated a new vaccine which can improve the effectiveness of the existing BCG. The new approach, which has been developed by Helen McShane and her colleagues, involves using a strain of vaccinia virus (cow pox) to which has been added a gene from TB. When the virus is administered to an individual, the cells which become infected also express the TB gene, provoking the immune system to respond by simultaneously developing an anti-TB response. As a result the vaccinee becomes immune to TB and smallpox ! Helen McShane joins us to describe why TB is a huge global problem, and how the new vaccine works :
Helen - TB is a bacterium that causes two million deaths each year and is widespread in the developing world. Those with HIV are even more susceptible, making TB a big problem. The reason tuberculosis is so common is that it is highly infectious. Some people with the disease develop abscesses in their lungs, meaning that every time they cough, they also cough up the bacterium. Treatment usually lasts about 6 months, but many people stop taking the drugs after one month when they start to feel better. This often results in the disease coming back or drug resistant strains of the disease emerging. Other people, especially in the developing world, do not take all the necessary drugs because they are too expensive. This further prevents TB from being eradicated.
Chris - So how do we currently protect ourselves from TB?
Helen - In Britain, children are given a BCG injection at the age of thirteen or fourteen. Studies have shown that this has been successful in reducing the incidence of tuberculosis in this country. In the developing world, BCG is administered at birth. This offers some protection against disseminated TB (TB spread outside of the lungs) in the first ten years of life, but doesn't offer any great protection from lung disease in early adulthood. A second BCG injection later in life makes no difference to a person's level of immunity. In fact, BCG in the developing world offers more protection against leprosy than TB!
Chris - I've spent some time working in London around TB patients. Does this mean that I have some TB bacterium in me that my immune system has managed to keep under control?
Helen - Yes, it's possible. Only one in ten people exposed to TB will go on to develop the disease. The rest will either get rid of the bacterium or have what is known as latent infection. This is when a person carries a disease but does not show any symptoms. However, the disease can be reactivated if you become immuno-suppressed, such as by taking drugs, becoming old, or contracting HIV. A third of the world are infected with TB in this way (about 2 billion people), although in Britain, latent infection is much lower. The levels of TB are higher in London, which is mostly to do with a higher and denser population.
Chris - So now we understand the problems with TB, what have you done to try and sort them out?
Helen - We have developed a vaccine which boosts the immune response to TB as well as improving the efficacy of BGC. As BCG already provides protection in the first ten years of life, it seemed unethical to replace it, so we have tried to improve it instead. The vaccine is based on the smallpox vaccine and also contains one protein from TB. Trials have not only shown the vaccine to be very safe, but if a person has already had a BCG injection, it stimulates a stronger immune response that lasts longer. This is very encouraging but is the first step on along path.
Chris - How are you measuring the effectiveness of the vaccine?
Helen - At the moment, we are measuring the immune response induced by the vaccine. TB lives inside cells and so we need a particular type of cellular immunity involving T-cells rather than antibodies. Our vaccine is a very strong stimulator of T-cells. Ultimately, we need to test it in the field, and we hope to do this in South Africa in two or three years time, after further preliminary studies. The experiment would involve giving half of the people in the study both the BCG and the new vaccine, whilst the other half would receive only the BCG. We would then follow these people for three years to see how many contract TB. An added benefit of the new vaccine is, of course, that those who receive it will also become immune to smallpox!